As the instruments to measure the temperature of hot molten metal such as molten pig iron and molten steel, radiation thermometers, optical pyrometers, thermocouple thermometers and etc. are used, and among them, the thermocouple thermometers are most preferable in terms of accuracy. Most commonly used thermocouple thermometers for measuring the temperature of molten pig iron and molten steel are consumable thermometers having a heat sensing portion that comprises a platinum-rhodium-based thermocouple (hereinafter referred to as the PR thermocouple) and a quartz glass tube to protect the tip of the thermocouple. When this thermometer is immersed in molten pig iron or molten steel, the heat sensing portion consumes and spoiled in a very short time (10 to 20 seconds) so that the temperature measurement must be finished in a short time. In addition, the heat sensing portion must be replaced for each measurement. Namely, this kind of thermometer cannot continuously measure a temperature for a long interval, and it is expensive to measure temperatures frequently. Moreover, the characteristics of each thermocouples differs from one another, and it is difficult to maintain the accuracy of temperature measurement. Therefore, there is a strong demand for a thermometer that can measure the temperature of molten metal continuously for a long interval. An immersing type thermocouple thermometer with a protecting tube is disclosed in, for example, Japanese Unexamined Utility Model Publication No. 38290/1980. This discloses to use an outer and an inner protecting tubes. Where the outer protecting tube, which contacts with a molten metal, is made of boron nitride ceramics and the inner protecting tube is made of alumina ceramics. Another known protecting tube is made of a composite material of molybdenum and zirconia.
A thermocouple thermometer employing such a protecting tube can be used to measure the temperature of molten pig iron or molten steel continuously for five to ten hours, but the service life of the protective tube is still insufficient. Recently, several materials having preferable characteristics for the protecting tube have become available. These materials have a good corrosion resistance against molten metal such as molten pig iron and molten steel and against molten slag, as well as a sufficient strength, heat resistance and heat conductivity. They are metal-boride-based ceramics such as ZrB.sub.2, TiB.sub.2 and TaB.sub.2. In particular, ZrB.sub.2 has the most preferable characteristics.
A thermometer employing a protecting tube made of metal-boride-based ceramics is disclosed in Japanese Unexamined Utility Model Publication No. 189629/1987.
The metal-boride-based ceramics is a non-oxide material, and a protecting tube made thereof, therefore, is oxidized by a surrounding oxidising atmosphere when used at a high temperature, to generate a reducing gas inside the protecting tube. On the other hand, when the PR thermocouple is used as a temperature measuring sensor, it will be deteriorated by the reducing gas due to sublimation, recrystallization and alloying. When a combination of the protecting tube and the PR thermocouple is used to measure the PR temperature of molten metal such as molten pig iron and molten steel, the PR thermocouple quickly deteriorates to abruptly change an electromotive force or else the wires break. As the result, the temperature sensor cannot continue to measure the temperature in a short time, compared with the service life of protecting tube determined by the corrosion rate.
An object of the present invention is to protect the PR thermocouple to be deteriorated by the reducing gas to be generated in the protecting tube made of metal-boride-based ceramics when it is used at a high service temperature, and to provide a thermocouple temperature sensor that can continuously measure the temperature of molten metal such as molten steel and molten pig iron for a long interval, and a method of measuring the temperature of molten iron with the thermocouple temperature sensor.